The evolution of the Earth's first animals more than 500 million years ago caused global warming, according to a study.
The research, published in the journal Nature Communications, found that 520-540 million years ago, animal life evolved in the ocean and began breaking down organic material on the seafloor, leading to more carbon dioxide and less oxygen in the atmosphere.
In the 100 million years that followed, conditions for these earliest animals became much harsher, as ocean oxygen levels fell and carbon dioxide caused global warming, researchers said.
"Like worms in a garden, tiny creatures on the seabed disturb, mix and recycle dead organic material - a process known as bioturbation," said Tim Lenton, a professor at the University of Exeter in the UK.
"Because the effect of animals burrowing is so big, you would expect to see big changes in the environment when the whole ocean floor changes from an undisturbed state to a bioturbated state," said Lenton.
"We did indeed see a decrease in oxygen levels in the ocean around 520 million years ago," Professor Filip Meysman, from the University of Antwerp in Belgium added.
"But evidence from the rock record showed sediment was only a little disturbed," said Meysman.
"The critical factor was to realise that the biggest changes happen at the lowest levels of animal activity. This meant that the first bioturbators had a massive impact," said Sebastiaan van de Velde, from the Vrije Universiteit Brussel in Belgium.
The researchers said this realisation was the "missing piece of the puzzle," and allowed them to construct a mathematical model of Earth around that time to look to the changes caused by these early life forms.
"When we ran our model, we were surprised by what we saw," said Benjamin Mills, from the University of Leeds in the UK.
"The evolution of these small animals did indeed decrease the oxygen in the ocean and atmosphere, but also increased atmospheric carbon dioxide levels to such an extent that it caused a global warming event," said Mills.
"We knew that warming occurred at this point in Earth history, but did not realise it could be driven by animals," he said.
The process made conditions worse for these animals, which possibly contributed to a number of mass extinction events during the first 100 million years of animal evolution, researchers said.
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